1. Field of the Invention
The present invention relates to a die for molding disc substrates such as CD-ROM, CD-R, DVD-ROM, DVD-RAM, DVD-R substrates and so on and, in particular, it relates to a stamper and an inner stamper holder.
2. Description of the Related Art
Conventionally, in a die for molding disc substrates, an inner stamper holder for holding an inner circumferential side of a stamper, of the construction as shown in [0003] and FIGS. 16 and 17 of Japanese Unexamined Patent Publication No. H05-54427, is typically used. Thus, the inner circumferential side of the stamper is held to a mirror plate by a claw portion of the inner stamper holder that is projected from a surface of the stamper toward a cavity. However, the inner stamper holder, shown in FIGS. 16 and 17 of Japanese Unexamined Patent Publication No. H05-54427 mentioned above, has a problem in that the flow of melted resin from a sprue toward an information recording surface of the stamper in the cavity is restricted by the claw portion mentioned above, which reduces the cross-sectional area of the cavity.
In order to solve this problem, in Japanese Unexamined Patent Publication No. H05-54427 mentioned above, as shown in [0030], [0031] and FIG. 8 thereof, the inner circumferential side of the stamper is shaped to diverge in a tapered fashion by an angle ranging from 20° to 70° and the claw portion of the inner stamper holder is shaped accordingly. By shaping these elements as described above, there is an effect that the claw portion is prevented from being projected from the information recording surface of the stamper toward the cavity so that the flow of the melted resin is not obstructed. Further, Japanese Unexamined Patent Publication No. H05-54427 describes that a combination of the inner circumferential side of the stamper and the claw portion of the inner stamper holder, shown in [0032] and FIG. 9 thereof, also has an effect similar to that described above.
Still further, as examples similar to those described above in Japanese Unexamined Patent Publication No. H05-54427, examples shown in [0013] and FIG. 4 of Japanese Unexamined Patent Publication No. H10-302328 and in FIGS. 5 and 6 of Japanese Unexamined Utility Model Publication No. S62-80620, are known.
In each of the examples described above in Japanese Unexamined Patent Publication No. H05-54427, Japanese Unexamined Patent Publication No. H10-302328 and Japanese Unexamined Utility Model Publication No. S62-80620, the inner circumferential side of the stamper must be formed to a specific shape. Further, considering the fact that the stamper is moved slightly because of the difference of thermal expansion between the stamper and the inner stamper holder and the pressure of the melted resin when the disc substrates are molded, as the inner circumferential side of the stamper is fitted completely under the claw portion of the inner stamper holder in the examples described above, there is a problem in that strength of the inner circumferential side of the stamper becomes insufficient and that the service life of the stamper is shortened.
Further, devices for forming a center hole of the stamper shown in FIGS. 1 and 4 of Japanese Registered Utility Model No. 3032851, which form configurations around the center hole of the stamper as shown in FIGS. 3(b) and 5(b) thereof, are known. However, it was thought absolutely impossible to hold the stamper so that the claw portion of the inner stamper holder is not projected toward the cavity and without reshaping the configurations around the center hole of the stamper and, therefore, no attempt has been made to address this problem.